1. Field of the Invention
This invention relates to high temperature, high strength magnetic alloys with high saturation magnetization useful for applications such as rotors, stators and/or magnetic bearings of an auxiliary power unit of an aircraft jet engine.
2. State of the Art
In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
Binary iron-cobalt (Fexe2x88x92Co) alloys containing 33-55% cobalt (Co) are extremely brittle due to the formation of an ordered superlattice at temperatures below 730xc2x0 C. The addition of about 2% vanadium (V) inhibits this transformation to the ordered structure and permits the alloy to be cold-worked after quenching from about 730xc2x0 C. The addition of V also benefits the alloy in that it increases the resistivity, thereby reducing the eddy current losses.
Fexe2x88x92Coxe2x88x92V alloys have generally been accepted as the best commercially available alloy for applications requiring high magnetic induction at moderately high fields. V added to 2 wt. % has been found not to cause a significant drop in saturation and yet still inhibit the ordering reaction to such an extent that cold working is possible.
However, conventional Fexe2x88x92Coxe2x88x92V alloys employing less than 2% by weight vanadium have undesirable inherent properties. For example, when the magnetic material undergoes a large magnetic loss the energy efficiency of the magnetic material deteriorates significantly. In addition, conventional Fexe2x88x92Coxe2x88x92V alloys exhibit certain unsuitable magnetic properties when subjected to rapid current fluctuations. Further, as the percentage of V exceeds 2 wt. %, the DC magnetic properties of the material deteriorate.
In a common form, the composition of Fexe2x88x92Coxe2x88x92V soft magnetic alloys exhibit a balance between favorable magnetic properties, strength, and resistivity as compared to magnetic pure iron or magnetic silicon steel. These types of alloys are commonly employed in devices where magnetic materials having high saturation magnetic flux density are required. Fexe2x88x92Coxe2x88x92V alloys have been used in a variety of applications where a high saturation magnetization is required, i.e. as a lamination material for electrical generators used in aircraft and pole tips for high field magnets. Such devices commonly include soft magnetic material having a chemical composition of about 48-52% by weight Co, less than about 2.0% by weight V, incidental impurities and the remainder Fe.
U.S. Pat. No. 4,647,427 to Liu discloses examples of Fexe2x88x92Coxe2x88x92V alloys containing long range order for enhanced mechanical properties. The alloys include, in wt. %, about 16% Fe, 22-23% V, 0-10% Ni, additions (0.4-1.4% Ti, Zr, or Hf, 0.5% Al, 0.5% Ti+0.5% Al, 0.9% Ti+0.5% Al, 3.2% Nb, and 0.8% Ti+1.2% Nb+0.4% Ce), and balance Co. The ordered lattice of this alloy imparts improved strength, including an inverse relationship for yield strength as a function of temperature. Titanium (Ti) is substituted for V to improve the mechanical properties, and niobium (Nb) is added for improved creep properties.
U.S. Pat. No. 4,933,026 to Rawlings et al. discloses soft magnetic cobalt-iron alloys containing V and Nb. The alloys include, in wt. %, 34-51% Co, 0.1-2% Nb, 1.9% V, 0.2-0.3% Ta, or 0.2% Ta+2.1% V. Rawlings et al. also mentions previously known magnetic alloys containing 45-55% Fe, 45-55% Co and 1.5-2.5% V. The objective of the alloy of Rawlings et al. is to obtain high saturization magnetization combined with ductility. The ductility and magnetization of the alloy of Rawlings et al. is attributed to the addition of niobium (Nb). Additionally, Rawlings et al. mentions the use of such an alloy in applications such as pole tips and aerospace applications.
U.S. Pat. No. 5,252,940 to Tanaka discloses an Fexe2x88x92Co alloy having a 1:1 ratio of Fe to Co and containing 2.1-5% V. The Fexe2x88x92Coxe2x88x92V composition of Tanaka provides high energy efficiency under fluctuating DC conditions by reducing eddy currents.
FeCoV alloys are disclosed in U.S. Pat. Nos. 3,634,072; 3,891,475; 3,977,919; 4,116,727; 4,933,026; 5,067,993; 5,252,940; 5,501,747; 5,741,374; and 5,817,191, the disclosures of which, as they are related to thermomechanical precessing of such alloys, are hereby incorporated by reference.
According to an article by Phillip G. Colegrove entitled xe2x80x9cIntegrated Power Unit for a Moore Electric Airplanexe2x80x9d, AIAA/AHS/ASEE Aerospace Design Conference, Feb. 16, 1993, Irvine, Calif., an integrated power unit provides electric power for main engine starting and for in-flight emergency power as well as for normal auxiliary power functions. Such units output electric power from a switched-reluctance starter-generator driven by a shaft supported by magnetic bearings. The starter-generator is exposed to harsh conditions and environment in which it must function, e.g., rotational speeds of 50,000 to 70,000 rpm and a continuous operating temperature of approximately 500xc2x0 C. The machine rotor and stator can be composed of stacks of laminations, each of which is approximately 0.006 to 0.008 inches thick. The rotor stack can be approximately 5 inches in length with a diameter of approximately 4.5 inches and the stator outside diameter can be about 9 inches. HiSat-50, an alloy produced by Telcon Metal Limited of England has been proposed for the rotor and stator laminations annealed at a temperature providing a desirable combination of strength and magnetic properties. The magnetic bearings are operated through attraction, rather than repulsion, of the shaft toward the magnetic force generator, the bearings exhibiting a desirable combination of bearing stiffness, load capability, allowable operating temperature and operational frequency. The operational temperature of the bearings can be on the order of 650xc2x0 F.
Iron-cobalt alloys have been proposed for magnetic bearings used in integrated power units and internal starter/generators for main propulsion engines according to an article by Richard T. Fingers et al. entitled xe2x80x9cMechanical Properties of Iron-Cobalt Alloys for Power Applications.xe2x80x9d Two iron-cobalt alloys investigated include Hiperco(trademark) alloy 50HS from Carpenter Technology Corporation and HS50 from Telcon Limited. After heat treating at 1300 to 1350xc2x0 F. for 1 to 2 hours, tensile properties were evaluated for specimens prepared from rolled sheet 0.006 inches thick. Both materials are categorized as near 50xe2x80x9450 iron-cobalt alloys having a B2-ordered microstructure but with small percentages of vanadium to increase ductility and other additions for grain refinement. Alloy 50HS is reported to include, in weight percent, 48.75% Co, 1.90% V, 0.30% Nb, 0.05% Mn, 0.05% Si, 0.01% C, balance Fe whereas HS50 includes 49.5% Co, 0.27% V, 0.45% Ta, 0.04% Mn, 0.08% Si, balance Fe. The alloys annealed at 1300xc2x0 F. are reported to exhibit the highest strength while those annealed at 1350xc2x0 F. produced the lowest strength. According to the article, in development of motors, generators and magnetic bearings, it will be necessary to take into consideration mechanical behavior, electrical loss and magnetic properties under conditions of actual use. For rotor applications these conditions are temperatures above 1000xc2x0 F. and exposure to alternating magnetic fields of 2 Tesla at frequencies of 500 Hz and the clamping of the rotor will result in large compressive axial loads while rotation of the rotor can create tensile hoop stresses of approximately 85 ksi. Because eddy current losses are inversely proportional to resistivity, the greater the resistivity, the lower the eddy current losses and heat generated. Resistivity data documented for 50HS annealed for 1 hour at temperatures of 1300 to 1350xc2x0 F. indicate a mean room temperature resistivity of about 43 micro-ohm-cm whereas a value of 13.4 micro-ohm-cm is reported for HS50 annealed for 2 hours at temperatures of 1300 to 1350xc2x0 F. The article concludes that both alloys appear to be good candidates for machine designs requiring relatively high strength and good magnetic and electrical performance.
Conventional soft magnetic alloys are used widely where high saturation magnetization values are important. However, their yield strengths are low at room temperature, and the strengths are even lower at high temperatures, making the alloys unsuitable for applications such as magnetic parts for jet engines that impose high temperatures and centrifugal stress on materials. Alloy design is critical for aerospace applications and becomes even more difficult when the magnetic requirements are imposed on the material along with the high temperature strength requirements. The room and high temperature strengths and high resistivity of the Fexe2x88x92Coxe2x88x92V alloys of the present invention overcome these and other deficiencies of conventional soft magnetic alloys.
An Fexe2x88x92Coxe2x88x92V alloy is provided in which the weight percent of constituents are such that (Fe+Co)xe2x89xa790%, (Fexe2x88x92Co)xe2x89xa710%, and 1.5 to 10% V. The alloy can be iron-based, cobalt-based, or have no base metal. Additional alloying constituents include B, C, Nb, Ti, W, Ni and/or Mo.